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CHAPTER 11 DATA ACCESS LAYER SERVICES
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It is not, however, suited as a general purpose approach to providing a data access layer for a distributed application of any significant size, scale, or complexity. Embedding structured query language in the presentation tier is not maintainable for larger applications. Typically you want to see data access encapsulated within its own layer of the application, probably only exposed by a business object layer that sits between the presentation and data layers. Ideally, it s best to create a layer of abstraction between UI developers and relational databases. This lets developers focus on meeting functional requirements, and relegates database modeling and schema to the information architects and DBAs, where it belongs. When these architectural requirements are present, the ObjectDataSource provides a much better solution.
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Another control that may be viable in a wider range of applications is the ObjectDataSource. This type provides similar functionality to a SqlDataSource, but it allows for binding to an object model instead of injecting SQL or stored procedure calls directly into the markup. This enables you to bind to, for example, a collection of business objects instead of the rows of a result set. Using this data source, you can have your pages declaratively bind to the middle tier of your application, instead of directly to the data tier. More specifically, these methods enable you to bind to methods rather than bind directly to database objects. These objects do need to be designed in a specific manner so that they ll work correctly with the ObjectDataSource. This data source control does use a true late binding strategy, but there are a few rules for how the methods it binds to must be formed. For example, the SelectMethod must return a type that the data-binding engine can bind to. This means if you have your own custom CustomerCollection type, you cannot bind directly to that type; you must have another method that returns an instance of CustomerCollection. This subtle difference means you must create types (or at least methods on your types) that are tailored to the ObjectDataSource control. Let s start with a very simple example. Here s the declaration of an ObjectDataSource that binds to a generic collection of BookDetail objects: <asp:ObjectDataSource ID="odsBookList" runat="server" SelectMethod="GetBookList" TypeName="BookBinding" />
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Note The code for the ObjectDataSource demo can be found in EditBook.aspx, EditBook.aspx.cs,
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BookBinding.cs, and BookDetails.cs, all of which are in the Web11 project.
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This declaration names a type using the TypeName attribute, and a method on that type using the SelectMethod attribute. The data source control then uses Reflection and late binding to create an instance of the type and get the data when it s time to bind. Here s the first part of the code for the BookBinding type you re using with this ObjectDataSource control:
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CHAPTER 11 DATA ACCESS LAYER SERVICES
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public class BookBinding { public BookBinding() {} public SqlDataReader GetBookList() { String sql = "select BookId, Title From Book order by Title"; SqlConnection cn = new SqlConnection(WebStatic.ConnectionString); SqlCommand cm = new SqlCommand(sql,cn); cn.Open(); return cm.ExecuteReader(CommandBehavior.CloseConnection); } The default constructor is required so that the control can create an instance of the type. You could omit it from this code, because you have no custom constructors, but if you have a custom constructor on your type, you must explicitly add the default constructor, even if it does nothing (like the preceding constructor). The GetBookList serves as the select method of the binding operation. Here you re simply passing through a SqlDataReader. Using code like this, you re really using the ObjectDataSource to bind directly to a data access layer. This is still a vast improvement over putting the data access code directly in the markup, and it doesn t take much code do. You could call a stored procedure instead of using inline SQL, which would be even better. You can use this control to bind any of the controls listed in Table 11-2. Here you ll create a ListBox bound to the ObjectDataSource: <asp:ListBox ID="lbBookList" runat="server" DataSourceID="odsBookList" Width=600px DataTextField="Title" DataValueField="BookID" Rows=12 AutoPostBack="True" /> This renders a list of books displaying the title and carrying the BookID as the underlying value of the list items. It s really not much less code than it would take to do this in 1.x. The main differences here are that there s no code in the code-behind of our page (the BookBinding.cs file is in the app_code directory, but it could be in an external assembly), and the code that actually does the binding is contained in the ObjectDataSource control. In this example, that means the only thing you don t have to do is call DataBind. However, when you design and build an entire system using these controls, you ll likely get better reuse out of your object layer, and your individual page code will be much simpler. This strategy is also much more powerful when it comes to editing data, as you can bind to a business object layer and have your business rules enforced. Let s take a look at another declaration of an ObjectDataSource:
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